Wing Tip Device

ABSTRACT

A wing tip device for fixing to the outboard end of a wing, the wing defining a wing plane, the wing tip device comprising: an upper wing-like element projecting upwardly with respect to the wing plane and having a trailing edge; and a lower wing-like element fixed with respect to the upper wing-like element and having a root chord and a trailing edge, the lower wing-like element root chord intersecting with the upper wing-like element, and the lower wing-like clement projecting downwardly from the intersection, wherein the upper wing-like element is larger than the lower wing-like element and the trailing edge of the lower wing-like element is adjacent the trailing edge of the upper wing-like element at the intersection, and wherein an included angle between the upper and lower wing-like elements at the intersection is less than, or equal to, 160 degrees. Also, a wing with the wing tip device; an aircraft with the wing; a method of fitting, or retro-fitting, the wing tip device to a wing; a method of modifying an existing wing tip device; and a method of operating a wing with the wing tip device.

RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.13/805,406, filed Dec. 19, 2012, which is the National Phase ofInternational Application Number PCT/EP2011/061552, filed Jul. 7, 2011,and claims priority from British Application Number 1011843.8, filedJul. 14, 2010.

FIELD OF THE INVENTION

The present invention relates to a wing tip device for fixing to theoutboard end of a wing. Also, a wing with the wing tip device; anaircraft with the wing; a method of fitting, or retro-fitting, the wingtip device to a wing; a method of modifying an existing wing tip device;and a method of operating a wing with the wing tip device.

BACKGROUND OF THE INVENTION

A wing tip device is attached to the outboard end of a wing to reduceinduced drag on the wing. In the case of e.g. an aircraft wing this canlead to improved fuel efficiency and reduced carbon emissions. Wing tipdevices can take a variety of forms.

A winglet is a wing-like element, that extends from the wing tip. Awinglet may extend upwardly or downwardly from the wing tip. NASA TND-8260 entitled “A Design Approach and Selected Wind-Tunnel Results atHigh Subsonic Speeds for Wing-Tip Mounted Winglets”; Whitcomb, R. T.;1976 describes a wing tip device having a lower winglet (extendingdownwardly from the wing tip) forward of an upper winglet (extendingupwardly from the wing tip). The sizing of these tip devices arerecommended in NASA T M 81230 entitled “Effect of Winglets on theInduced Drag of Ideal Wing Shapes”; R T Jones and T A Lasinski 1980.

A wing tip fence is a special form of wing tip device that extendsvertically both above and below the wing tip. U.S. Pat. No. 4,714,215describes a wing tip fence.

Another example of a wing tip device is a non-planar wing tip extension,i.e. it extends out of the plane of the wing to which it is attached, Awinglet may be considered to be a particular example of a non-planarwing tip extension. US 2002/0162917 describes a non-planar wing tipextension having continuously increasing curvature of local dihedral,continuously increasing sweepback (at both leading and tailing edges),and continuously decreasing chord in the outboard direction.

A winglet may include a substantially planar portion joined to the wingtip by a curved transition portion to form a blended winglet, such asdescribed in U.S. Pat. No. 5,348,253. The transition portion has aconstant radius of curvature. The specified blend is said to reduceinterference drag effects at the wing tip.

Alternatively, a winglet may include a substantially planar portionjoined to the wing tip by a non-planar wing tip extension portion, suchas described in WO 2008/061739. The non-planar wing tip extensionportion has increasing curvature of local dihedral in the outboarddirection. The wing tip extension portion is said to further reduceinterference drag effects compared to a blended winglet with a constantradius transition.

Another example of a wing tin device is a substantially planar wing tipextension, such as the raked vying tin described in U.S. Pat. No.6,089,502, which docs not extend substantially out of the plane of thewing. Raked wing tips can achieve similar drag reduction performance towinglets.

Span constraints on aircraft, due to e.g. airport compatibility gatelimits or aircraft category flying constraints, mean that winglets ornon-planar wing tip extensions, rather than raked wing tips, may need tobe adopted in order to reduce induced drag on the wing. Since winglets(and non-planar wing tip extensions more generally) extend out of theplane of the wing to which they are attached, an effective increase inthe wing aspect ratio can be achieved (which reduces the vortex-induceddrag on the wing) without significantly increasing wing span.

The problem of span constraints is traditionally solved by optimisingthe wing span of the aircraft in the (full fuel load) ground shape wherethe span constraints are applicable. However, due to the bend induced onthe wing shape from aeroelastic effects during flight, the wing span ofthe resulting flight shape is usually reduced and is therefore no longeroptimal. This traditional approach therefore carries a performanceshortfall. This problem becomes even more appreciable with greater useof relatively flexible wings to reduce structural weight, which tend toresult in increased wing bending under aerodynamic load when compared tomore rigid designs.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a wing tip device for fixing tothe outboard end of a wing, the wing defining a wing plane, the wing tipdevice comprising: an upper wing-like element projecting upwardly withrespect to the wing plane and having a trailing edge; and a lowerwing-like element fixed with respect to the upper wing-like element andhaving a root chord and a trailing edge, the lower wing-like elementroot chord intersecting with the upper wing-like element, and the lowerwing-like element projecting downwardly from the intersection, whereinthe upper wing-like clement is larger than the lower wing-like elementand the trailing edge of the lower wing-like element is adjacent, thetrailing edge of the upper wing-like element at the intersection, andwherein an included angle between the upper and lower wing-like elementsat the intersection is less than, or equal to, 160 degrees.

A second aspect of the invention provides a wing having an outboard endand a wing tip device in accordance with the first aspect fixed to itsoutboard end.

A third aspect of the invention provides an aircraft having a wingaccording to the second aspect.

A fourth aspect of the invention provides a method of fitting, orretro-fitting, a wing tip device to a wing, the method comprising fixinga wing tip device in accordance with the first aspect to the outboardend of a wing.

A fifth aspect of the invention provides a method of modifying a wingtip device fixed to, or for fixing to, the outboard end of a wing, thewing defining a wing plane, the existing wing tip device comprising anupper wing-like element projecting upwardly with respect to the wingplane and having a trailing edge, and the method comprising providing alower wing-like element smaller than the upper wing-like element andhaving a root chord and a trailing edge, and fixing the lower wing-likeelement to the upper wing-like element such that: the lower wing-likeelement root chord intersects with the upper wing-like element, and thelower wing-like element projects downwardly from the intersection; andthat the trailing edge of the lower wing-like element is adjacent thetrailing edge of the upper wing-like element at the intersection; andthat an included angle between the upper and lower wing-like elements atthe intersection is less than, or equal to, 160 degrees.

A sixth aspect of the invention provides a method of operating a winghaving a wing tip device fixed to the outboard end of the wing, the wingdefining a wing plane, and the wing tip device comprising: an upperwing-like element projecting upwardly with respect to the wing plane andhaving a trailing edge; and a lower wing-like element fixed with respectto the upper wing-like element and having a root chord and a trailingedge, the lower wing-like element root chord intersecting with the upperwing-like element, and the lower wing-like element projecting downwardlyfrom the intersection, wherein the upper wing-like element is largerthan the lower wing-like element and the trailing edge of the lowerwing-like element is adjacent the trailing edge of the upper wing-likeelement at the intersection, and wherein an included angle between theupper and lower wing-like elements at the intersection is less than, orequal to, 160 degrees, and the method comprising subjecting the wing toaerodynamic loads such that the wing shape undergoes aeroelasticdeformation to a state in which wing bending causes rotation of the wingtip device about the wing root such that the tip of the lower wing-likeelement extends further outboard in the spanwise direction than the tipof the upper wing-like element.

The invention is advantageous in that the lower wing-like element actsto offset at least some of the decrease in wing span that occurs due toaeroelastic deformation in the flight shape, whilst the upper and lowerwing-like elements may still be optimised to meet any applicable spanconstraints in the ground shape. The addition of the lower element to awing tip device comprising only an upper wing-like element (e.g. awinglet) has been shown to reduce drag on the wing/wing tip devicecombination by around a further 1.9% overall, with a vortex dragreduction of around a further 25 to 40% relative to that provided by theupper element alone.

Near coincidence of the upper and lower element trailing edges isimportant to avoid wake disturbance effects. The trailing edges need notbe exactly coincident but must be adjacent so as to avoid the wake ofone element impacting on the flow over the other element at theintersection.

The included angle between the upper and lower wing-like elements at theintersection is important such that the lower element provides anincrease in span in the flight shape. The cant angle of the lowerelement (i.e. the angle between the vertical x-z plane and the element)may be optimised in order to achieve the maximum span increase in theflight shape, with due consideration to minimising interference effectsat the intersection. Note that a wing tip fence has an approximately 180degree included angle between vertical upper and lower elements, and sothe lower element provides negligible increase in span in the flightshape.

The upper wing-like element is larger than the lower wing-like element.The lower wing-like element may have an element plan form area less thanapproximately 25% of the upper wing-like element planform area. Notethat the planform area of each element is viewed in a plane different tothat of the wing planform area. The planform area of the lower elementmay be designed in order to deliver the required span loading whileminimising the cruise viscous drag penalty, and to provide good lowspeed high lift performance. Ground height clearance constraints maylimit the size of the lower element.

The lower wing-like element is fixed with respect to the upper wing-likeelement. The wing tip device is fixed with respect to the wing. Theinvention is not concerned with moveable wing tip devices as these aregenerally heavier than fixed devices, which may offset any performancebenefit. Also, solving the problem of span constraints is somewhattrivial with moveable wing tip devices.

An included angle between the wing plane and the lower wing-like elementmay be at least 110 degrees. The lower element therefore extendsoutboard from the outboard end of the wing, and interference effectsbetween the wing lower surface and the lower element can be minimised.

The included angle between the upper and lower wing-like elements at theintersection may be at least 80 degrees, and preferably is at least 90degrees. This helps minimise interference effects between the upper andlower elements at the intersection.

The lower wing-like element may be substantially planar.

Alternatively, the lower wing-like element may be substantiallynon-planar. In particular, the lower element may have wing twist, e.g.wash-out. The lower element may have a spanwise curvature of increasinganhedral from root to tip.

The lower element may have a toe angle relative to the vertical x-zplane.

The lower element may have a sweep back angle. In particular, the lowerelement may have a swept back leading edge. The sweep back angle of thelower element leading edge may be similar to that of the upper element.

The upper wing-like element may include a substantially planar portion.

In one embodiment, the upper wing-like element may be substantiallyplanar. The upper element may be a winglet.

In another embodiment, the upper wing-like element may include asubstantially planar portion and an arcuate transition portion adaptedto smoothly blend the outboard end of the wing into the substantiallyplanar portion of the upper wing-like element. The upper element may bea blended winglet. The transition portion may have a constant radius ofcurvature. The blend helps to reduce interference drag effects at thewing tip.

In yet another embodiment, the upper wing-like element may include asubstantially planar portion and a non-planar curved wing tip extensionadapted to smoothly blend the outboard end of the wing into thesubstantially planar portion of the upper wing-like element. The upperelement may be a winglet blended into the wing by a non-planar wing tipextension portion. The non-planar wing tip extension portion may haveincreasing curvature of local dihedral in the outboard direction. Thewing tip extension portion helps to further reduce interference drageffects compared to a blended winglet with a constant radius transition.

The upper wing-like element may be a substantially non-planar curvedwing tip extension. The extension may have continuously increasingcurvature of local dihedral, continuously increasing sweepback (at bothleading and tailing edges), and continuously decreasing chord in theoutboard direction.

The upper wing-like element may have wing twist from root to tip, e.g.wash-out.

The upper wing-like element may have a toe angle relative to thevertical x-z plane.

The upper wing-like element may have a sweep back angle. In particular,the upper element may have a swept hack leading edge. The sweep backangle of the upper element leading edge may be similar to that of thelower element.

The intersection between the lower wing-like clement and the upperwing-like element may be at the outboard end of the wing.

Alternatively, the intersection between the lower wing-like element andthe upper wing-like element may be outboard of the outboard end of thewing. This may be particularly advantageous where the upper element issmoothly blended into the outboard end of the wing. In this case, theintersection may be on the lower surface of the upper element.

The root chord of the lower element may extend along only pan of thelocal chord of the upper clement at the intersection.

When the aircraft is on the ground and the wing is subjected to downwarddeflection due to full fuel load, the tip of the lower wing-like elementmay extend no further outboard in the span wise direction than the tipof the upper wing-like element. In this way both upper and lower elementtips may be at the airport gate limit, for example.

When the aircraft is on the ground and the wing is subjected to downwarddeflection due to full fuel load, the spanwise extent of the tip of thelower wing-like element may be substantially equal to the spanwiseextent of the lip of the upper wing-like clement. Alternatively, thespanwise extent of the tip of the lower wing-like element may be greaterthan the spanwise extent of the tip of the upper wing-like element whenthe span of the tip of the upper element is substantially less than theairport gate limit

When the aircraft is in flight, the tip of the lower wing-like elementmay extend further outboard in the spanwise direction than the tip ofthe upper wing-like element due to aeroelastic deformation of the wingshape.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of she invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 illustrates a prior art aircraft wing with an upper winglet,shown a) in its ground shape, and b) in its flight shape;

FIG. 2 illustrates Detail A of FIG. 1 showing the span-limit on theground, and the loss of span due to wing deformation under aerodynamicload;

FIG. 3 illustrates an aircraft wing/wing tip device according to a firstembodiment having a planar upper winglet and a planar lower winglet,shown a) in its ground shape, and b) in its flight shape, andillustrating the span gained front the lower element in the flightshape;

FIG. 4 illustrates the aircraft wing/wing tip device (in the groundshape) according to the first embodiment in detail;

FIGS. 5 and 6 illustrates graphically the further reduction in drag dueto the lower element in the first embodiment;

FIG. 7 illustrates an aircraft wing/wing tip device (in the groundshape) according to a second embodiment having a planar upper wingletand a non-planar lower winglet;

FIG. 8 illustrates an aircraft wing/wing tip device (in the groundshape) according to a third embodiment having a blended upper wingletand a planar lower winglet;

FIG. 9 illustrates an aircraft wing/wing tip device (in the groundshape) according to a fourth embodiment having an upper winglet blendedinto the wing with a non-planar wing tip extension, and a planar lowerwinglet (although a non-planar lower winglet may also be applied); and

FIG. 10 illustrates a perspective view of the wing/wing tip device ofthe fourth embodiment;

FIG. 11 illustrates a plan view of the wing/wing tip device of thefourth embodiment;

FIG. 12 illustrates an aircraft wing/wing tip device according to afifth embodiment having a non-planar (upper) wing tip extension and aplanar lower winglet; and

FIG. 13 illustrates a perspective view of the wing tip device of thefifth embodiment.

DETAILED DESCRIPTION OF EMBODIMENT(S)

FIG. 1 illustrates a prior art aircraft wing 1 having an inboard wingroot 2 and an outboard wing tip 3. A wing tip device comprising anupwardly extending winglet 4 is fixed to the outboard end 3 of the wing1. The wing 1 is shown in FIG. 1 in a) its ground shape (i.e. with theaircraft on the ground and with a full fuel load in the wing), and b)its flight shape (i.e. with deformation due to aerodynamic load).

FIG. 2 illustrates Detail A of FIG. 1 and the broken line 5 illustratesa span constraint imposed on the aircraft due to e.g. airportcompatibility gate limits or aircraft category flying constraints. Thespan limit 5 is applicable for the ground shape shown in FIG. 2 a). FIG.2 b) illustrates the loss 6 in wing span due to wing deformation in theflight shape. This loss in span 6 may be up to around 3%.

FIG. 3 illustrates an aircraft wing 101 according to a first embodimenthaving a planar upper winglet 104 and a planar lower winglet 107. Theupper winglet 104 is fixed to the outboard end 103 of the wing 101. Thewing 101 defines a wing plane 108. The upper winglet 104 projectsupwardly with respect to the wing plane 108. The upper winglet 104 has atip 109 and a root 110. The lower winglet has a tip 111 and a root 112.The lower winglet root chord 112 intersects with the upper winglet 104and the lower winglet 107 projects downwardly from this intersection.The upper and lower winglets 104, 107 each have a leading edge and atrailing edge and the trailing edges are adjacent at the intersection.FIG. 3 a) illustrates the wing 101 in its ground shape where the tip 109of the upper winglet 104 and the tip 111 of the lower winglet 107 arecoincident at the span limit 105. FIG. 3 b) illustrates the wing 101 inits deformed flight shape and shows how a potential loss in span 106 dueto the upper winglet 104 is mitigated by an increase in span 113 gainedfrom the lower winglet 107. This gain in span 113 due to the lowerwinglet 107 is approximately 2%.

FIG. 4 illustrates the aircraft wing 101 of the first embodiment ingreater detail. The lower winglet 107 is sized and oriented so as tomaximise the span increase in the flight shape, whilst minimisinginterference effects at the intersection between the lower winglet 107and the upper winglet 104. In addition, a ground clearance height Gbetween the ground and the tip 111 of the lower winglet 107 is takeninto account. The resultant geometry provides an included angle betweenthe upper and lower wing elements of around 132°, and an included anglebetween the wing plane 108 and the lower winglet 107 of around 128°. Thelower winglet 107 has a winglet planform area of around 20% of the upperwinglet 104 planform area. The relatively small size of the lowerwinglet 107 minimises the viscous drag penalty at cruise whilstdelivering the required optimum span loading.

FIGS. 5 and 6 illustrate graphically the effect of the addition of thelower winglet element 107 on the lift and vortex drag characteristics ofthe wing 101. In FIGS. 5 and 6 the line with circular markers representsa reference wing corresponding to the wing 101 with a tip near to animposed span limit without any wing tip device. The line with the crossmarkers illustrate the wing 101 with only the upper winglet element 104(sized as recommended in NASA T M 81230 entitled “Effect of Winglets onthe Induced Drag of Ideal Wing Shapes”; R T Jones and T A Lasinski1980), and the line with the triangular markers represent the wing 101with both the upper and lower winglet elements 104, 107. FIG. 5illustrates the relationship between lift and drag coefficients (CL, CD)and shows an improvement in lift to drag ratio for the wing 101 withboth the upper and lower winglet element 104, 107 as compared to boththe reference wing and the wing with only an upper winglet element. FIG.6 illustrates a drag saving due to the addition of the lower wingletelement 107 of around 1.9% at the mid-cruise weight lift coefficient(CL=0.5) relative to the wing with upper element 104 alone. The vortexdrag reduction provided by the lower winglet element 107 is a furtherreduction of around 25 to 40%.

FIG. 7 illustrates an aircraft wing 201 according to a second embodimenthaving a planar upper winglet 204 and a non planar lower winglet 207.The wing 201 defines a wing plane 208 and the upper winglet 204 projectsupwardly with respect to the wing plane 208. The upper winglet 204 isfixed to the outboard end 203 of the wing 201. The lower winglet 207 hasa root chord 212 which intersects with the upper winglet 204. The lowerwinglet 207 projects downwardly from the intersection. The upper winglet204 has a tip 209 and a root 210. The lower winglet 207 has a tip 211that is coincident in the spanwise direction with the tip 209 at thespan limit 205. The upper and lower winglets 204, 207 each have aleading edge and a trailing edge and the trailing edges are adjacent atthe intersection. The wing 201 is illustrated in FIG. 7 in its groundshape where the span limit 205 is enforced.

The lower winglet 207 has increasing curvature of local anhedral fromroot 212 to tip 211. The lower winglet 207 may have a toe in on toe-outangle to optimise the low speed performance of the tip device.

The wingtip device for the wing 201 has been optimised so as to maximisethe span increase under flight aerodynamic loads, whilst minimisinginterference effects between the lower winglet 207 and the lower surfaceof the wing 201, and between the upper and lower winglets 204, 207. Theresultant optimised geometry has an included angle between the upper andlower winglets 204, 207 of around 120°, and an included angle betweenthe wing plane 208 and the lower winglet 207 of around 138°. In theflight shape, the lower winglet 207 provides a farther gain in span ascompared to the lower winglet 107 of the wing 101, principally due tothe increased root 212 to tip 211 height of the lower winglet 207 andthe flexibility of the lower winglet 207 which straightens under flightloads.

FIG. 8 illustrates an aircraft wing 301 according to a third embodimenthaving a blended upper winglet 304 and a planar lower winglet 307. Thewing 301 has an outboard end 303 to which is fixed the blended upperwinglet 304. The upper winglet 304 has a tip 309 and a root 310. Theupper winglet 304 is fixed to the outboard end 303 of the wing 301 byits root end 310. The upper winglet 304 has a substantially planarportion 314 and an arcuate transition portion 315. The transitionportion 315 is adapted to smoothly blend the outboard end 303 of thewing 301 into the substantially planar portion 314. The arcuatetransition portion 315 has a substantially constant radius of curvatureR.

The lower winglet 307 is fixed to the lower surface of the transitionportion 315 of the upper winglet 304. The lower winglet has a tip 311and a root 312. The root chord of the lower winglet 307 intersects withthe upper winglet 304 and the lower winglet projects downwardly from theintersection. The upper and lower winglets 304, 307 each have a leadingedge and a trailing edge and the trailing edges are adjacent at theintersection. The transition portion 315 helps reduce interferenceeffects between the substantially planar portion 314 and the wing 201.

The tip 309 of the upper winglet 304 is substantially coincident ii thevertical x-z plane with the tip 311 of the lower winglet 307 at the spanlimit 305. An included angle between the upper and lower winglets 304,307 at the intersection is around 84°. It is preferable that this angleis at least 80° so as to avoid interference effects between the upperand lower winglets 304, 307. Since the intersection is on the lowersurface of the blended transition portion 315, this angle is measuredbetween the transition portion lower surface tangent and the lowerwinglet 307. An included angle between the wing plane 308 and the lowerwinglet 307 is around 125°. The substantially planar portion 314 of theupper winglet 304 has a cant angle relative to the vertical x-z plane ofaround 7° to 15°.

The lower winglet element 307 has an element planform area ofapproximately 25% of the upper winglet element 304 planform area. Whilstthe lower winglet 307 is substantially planar it may have some wingtwist from root 312 to tip 311. The lower winglet 307 may additionallyor alternatively have a toe in or toe out angle to optimise low speedperformance. Similarly, the upper winglet 304 may have some twist andmay have a toe in or toe out angle. The lower winglet 307 has a sweephack angle and in particular the leading edge is swept back. The upperwinglet 304 is also swept back and has a swept back leading edge and aswept back trailing edge.

If ground clearance limits allow, then the lower winglet element 307could be replaced with a non-planar lower winglet element similar tothat described above with reference to FIG. 7.

FIG. 9 illustrates an aircraft wing/wingtip device combinationcomprising a wing 401 a blended upper winglet 404 and a planar lowerwinglet 407. The wing 401 has an outboard end 403 and defines a wingplane 408. The upper winglet 404 includes a substantially planar portion414 and a blended transition portion 415. The transition portion 415smoothly blends the outboard end 403 of the wing 401 into thesubstantially planar portion 414 of the upper winglet 404. Thetransition portion 415 is a non-planar curved wing tip extension havingcontinuously increasing curvature of local dihedral, continuouslyincreasing sweep back (at both leading and trailing edges) andcontinuously decreasing chord in the outboard direction. The non-planarcurved wing tip extension portion 415 provides improved drag performancefor the upper winglet 404 in comparison to the blended upper winglet 304shown in FIG. 8.

The upper winglet 404 has a root 410 and a tip 409. The substantiallyplanar portion 414 of the upper winglet 404 has a cant angle of around7° to the vertical x-z plane. A substantially planar lower winglet 407is fixed to the lower surface of the non-planar curved wing tipextension portion 415 of the upper winglet 404. The lower winglet 407has a tip 411 and a root 412. The root chord of the lower winglet 407intersects with the upper winglet 404 and the lower winglet projectsdownwardly from the intersection.

An included angle between the upper and lower winglets 404, 407 at theintersection is around 86°. Since the intersection is on the lowersurface of the non planar curved wing tip extension portion 415 of theupper winglet 404, this angle is measured from a local surface tangentto the lower surface of the non-planar curved, wing tip extensionportion 415 at the intersection. This included angle is preferablygreater than 80° to avoid interference effects between the upper andlower winglets 404, 407. An included angle between the wing plane 408and the lower winglet is around 124°. The tip 409 of the upper winglet404 is substantially coincident in the vertical x-z plane to the tip 411of the lower winglet 407 at the span limit 405.

FIGS. 10 and 11 illustrate perspective and plan views respectively ofthe wing/wingtip device combination of the fourth embodiment. From FIG.10 in particular it can be seen that the trailing edge 416 of the upperwinglet 404, and the trailing edge 417 of the lower winglet 407 aresubstantially adjacent at the intersection. The trailing edges 416, 417are sufficiently close that the wake from the lower winglet 407substantially does not interfere with the flow over the upper winglet404. The upper winglet 404 has a leading edge 418 that is sweptbackwards at d the lower winglet 407 also has a leading edge 419 that isswept backwards. The trailing edge 416 of the upper winglet 404 is sweptbackwards and the trailing edge 417 of the lower winglet 407 is alsoswept backwards.

In FIG. 11, the plan view (i.e. the top down view in the x-y plane)illustrates how the upper winglet 404 “shadows” at least part of thelower winglet 407. This is due to the coincidence of tips 409, 411 ofthe upper and lower winglets 404, 407 in the vertical x-z plane. As bestshown in FIG. 10, the root chord 412 of the lower winglet 407 occupiesonly part of the local chord of the upper winglet 404 at theintersection. Due to the near coincidence of the trailing edges 416, 417the leading edge 419 of the lower winglet 407 is positionedsubstantially aft of the leading edge 418 of the upper winglet 404.

FIG. 12 illustrates an aircraft wing/wingtip device combinationaccording to a fifth embodiment, comprising a wing 501 with a wingtipdevice comprising an upper non-planar wingtip extension 504 and a lowerplanar winglet 507. The wing 501 has an outboard end 503 and defines awing plane 508. The non-planar wingtip extension 504 has a root 510 anda tip 509 and is fixed to the outboard end 503 of the wing 501 by itsroot 510. The non-planar curved wingtip extension 504 has continuouslyincreasing curvature of local dihedral, continuously increasingsweepback (at both leading and trailing edges 518, 516), andcontinuously decreasing chord in the outboard direction, y.

The non-planar curved wingtip extension 504 is substantially non-planarfrom root 510 to tip 509. The tip 509 forms a cant angle ofapproximately 8° with the vertical x-z plane. The lower winglet 507 hasa tip 511 and a root 512 and the root chord intersects with thenon-planar curved wingtip extension 504, with the lower winglet 507projecting downwardly from the intersection. An included angle betweenthe non-planar wingtip extension 504 and the lower winglet 507 at theintersection is approximately 82°. This angle is measured between thelower winglet 507 and a local surface tangent to the lower surface ofthe non planar curved wingtip extension 504 at the intersection. Anincluded angle between the wing plane 508 and the lower winglet 507 isapproximately 126°. The tips 509, 511 of the non-planar curved wingtipextension 504 and the lower winglet 507 are substantially coincident inthe vertical x-z plane at the span limit 506.

FIG. 13 illustrates the wingtip device in accordance with the fifthembodiment in a perspective view and clearly shows that the trailingedge 516 of the non-planar curved wingtip extension 504 is substantiallycoincident with the trailing edge 517 of the lower winglet 507 at theintersection. Both the non-planar curved wingtip extension 504 and thelower winglet 507 have a sweepback angle and the leading and trailingedges 516, 517, 518, 519 each have a respective sweepback angle.

The lower winglet 507 may be only substantially planar and may featurewinglet twist from root to tip and a toe in or toe out angle relative tothe free stream flow. Similarly, the non planar curved wingtip extension504 may feature wing twist and a toe in or toe out angle relative to thefree stream flow. The lower winglet 507 may be replaced with asubstantially non-planar curved lower winglet, similar to that describedabove with reference to FIG. 7 if ground height clearance limits allow.

Each of the second to fifth embodiments described above with referenceto FIGS. 7 to 13 are shown with the respective wing/wingtip devicecombination in its ground shape. Due to aerodynamic loads on the wingduring flight, deformation of the wing will cause rotation of thewingtip device about the wing root such that the tip of the lowerwing-like element extends further outboard in the spanwise directionthan the tip of the upper wing-like element. The lower wing-like elementin each case therefore provides an increase in wing span when comparedto wingtip devices having only the upper wing-like element in each case.

The wingtip devices described in the first to fifth embodiments abovemay be fitted, or retro-fit to the outboard end of an aircraft winghaving either no wingtip device or as a replacement for an existingwingtip device. Furthermore, the lower wing-like element may be providedas a retro-fit modification to an existing wingtip device having only anupper wing-like element so as to form a wingtip device according to thisinvention.

Although the invention has been described above with reference to one ormore preferred embodiments, it will be appreciated that various changesor modifications may be made without departing from the scope of theinvention as defined in the appended claims.

What is claimed is:
 1. A wing tip device for fixing to the outboard endof a wing, the wing defining a wing plane, the wing tip devicecomprising: an upper wing-like element projecting upwardly with respectto the wing plane and having a trailing edge; and a lower wing-likeelement fixed with respect to the upper wing-like element and having aroot chord and a trailing edge, the lower wing-like element root chordintersecting with the upper wing-like element, and the lower wing-likeelement projecting downwardly from the intersection, wherein the upperwing-like element is larger than the lower wing-like element and thetrailing edge of the lower wing-like element is adjacent the trailingedge of the upper wing-like element at the intersection, wherein anincluded angle between the upper and lower wing-like elements at theintersection is less than, or equal to, 160 degrees, and wherein thelower wing-like element has an element planform area less thanapproximately 25% of the upper wing-like element planform area.
 2. Awing tip device according to claim L wherein an included angle betweenthe wing plane and the lower wing-like element is at least 110 degrees.3. A wing tip device according to claim 1, wherein the included anglebetween the upper and lower wing-like elements at the intersection is atleast 80 degrees.
 4. A wing tip device according to claim 1, wherein thelower wing-like element is substantially planar.
 5. A wing tip deviceaccording to claim 1, wherein the lower wing-like element issubstantially non-planar.
 6. A wing tip device according to claim 1,wherein the lower wing-like element has wing twist.
 7. A wing tip deviceaccording to claim 1, wherein the lower wing-like element has a sweepback angle.
 8. A wing tip device according to claim 1, wherein the upperwing-like element includes a substantially planar portion.
 9. A wing tipdevice according to claim 8, wherein the upper wing-like element issubstantially planar.
 10. A wing tip device according to claim 8,wherein the upper wing-like element further includes an arcuatetransition portion adapted to smoothly blend the outboard end of thewing into the substantially planar portion of the upper wing-likeelement.
 11. A wing tip device according to claim 8, wherein the upperwing-like element further includes a non-planar curved wing tipextension adapted to smoothly blend the outboard end of the wing intothe substantially planar portion of the upper wing-like element.
 12. Awing tip device according to claim 1, wherein the upper wing-likeelement is a substantially non-planar curved wing tip extension.
 13. Awing tip device according to claim 1, wherein the upper wing-likeelement has a sweep back angle.
 14. A wing tip device according to claim1, wherein the intersection between the lower wing-like element and theupper wing-like element is at the outboard end of the wing.
 15. A wingtip device according to claim 1, wherein the intersection between thelower wing-like element and the upper wing-like element is outboard ofthe outboard end of the wing.
 16. A wing having an outboard end and awing tip device in accordance with claim 1 fixed to its outboard end.17. An aircraft having a wing according to claim
 16. 18. A method offitting, or retro-fitting, a wing tip device to a wing, the methodcomprising fixing a wing tip device in accordance with claim 1 to theoutboard end of a wing.
 19. A method of modifying a wing tip devicefixed to, or for fixing to, the outboard end of a wing, the wingdefining a wing plane, the existing wing tip device comprising an upperwing-like element projecting upwardly with respect to the wing plane andhaving a trailing edge, and the method comprising providing a lowerwing-like element smaller than the upper wing-like element and having aroot chord and a trailing edge, and fixing the lower wing-like elementlo the upper wing-like element such that: the lower wing-like elementroot chord intersects with the upper wing-like element, and the lowerwing-like element projects downwardly from the intersection; and thatthe trailing edge of the lower wing-like element is adjacent thetrailing edge of the upper wing-like element at the intersection; andthat an included angle between the upper and lower wing-like elements atthe intersection is less than, or equal to, 160 degrees, and that thelower wing-like element has an element planform area less thanapproximately 25% of the upper wing-like element planform area.
 20. Amethod of operating a wing having a wing tip device fixed to theoutboard end of the wing, the wing defining a wing plane, and the wingtip device comprising: an upper wing-like element projecting upwardlywith respect to the wing plane and having a trailing edge; and a lowerwing-like element fixed with respect to the upper wing-like element andhaving a root chord and a trailing edge, the lower wing-like elementroot chord intersecting with the upper wing-like element, and the lowerwing-like element projecting downwardly from the intersection, whereinthe upper wing-like element is larger than the lower wing-like elementand the trailing edge of the lower wing-like element is adjacent thetrailing edge of the upper wing-like element at the intersection,wherein an included angle between the upper and lower wing-like elementsat the intersection is less than, or equal to, 160 degrees, and whereinthe lower wing-like element has an element planform area less thanapproximately 25% of the upper wing-like element planform area, and themethod comprising subjecting the wing to aerodynamic loads such that thewing shape undergoes aeroelastic deformation to a state in which wingbending causes rotation of the wing tip device about the wing root suchthat the tip of the lower wing-like element extends further outboard inthe spanwise direction than the tip of the upper wing-like element.